A PRACTICAL GUIDE TO SOLAR ARRAY SIMULATION AND PCDU TEST

Noah Schmitz (1), Greg Carroll (2), Russell Clegg (3)

(1) Agilent Technologies, 9780 S Meridian Blvd, Englewood CO, USA, noah_schmitz@agilent.com (2) Agilent Technologies, 900 South Taft Avenue, Loveland, CO, USA, greg_carroll@agilent.com

(3) Agilent Technologies, 550 Clark Drive, Suite 101, Budd Lake NJ, USA, russell_clegg@agilent.com

ABSTRACT

Solar arrays consisting of multiple photovoltaic segments provide power to satellites and charge internal batteries for use during eclipse. Solar arrays have unique I-V characteristics and output power which vary with environmental and operational conditions such as temperature, irradiance, spin, and eclipse. Therefore, specialty power solutions are needed to properly test the satellite on the ground, especially the Power Control and Distribution Unit (PCDU) and the Array Power Regulator (APR.) This paper explores some practical and theoretical considerations that should be taken into account when choosing a commercial, off- the-shelf solar array simulator (SAS) for verification of the satellite PCDU. An SAS is a unique power supply with I-V output characteristics that emulate the solar arrays used to power a satellite. It is important to think about the strengths and the limitations of this emulation capabilit y, how closely the SAS approximates a real solar panel, and how best to design a system using SAS as components. 1. EMULATION OF A SOLAR PANEL

It is important to consider both the strengths and the limitations of a commercial SAS when designing the Electronic Ground Support Equipment (EGSE) for PCDU verif ication. The main benefits of using a piece of test equipment rather than an actual solar panel are convenience and flexibilit y. The main drawbacks relate to the simple fact that any electronic device has fundamental li mits in dynamic performance. 1.1 Benefits Space is a hostile environment. A satellit e will encounter rapid, large-magnitude variations in irradiance and temperature, which affect performance and efficiency. These variations are caused by physical phenomena (such as distance from the sun) as well as operational phenomena (such as eclipse.) It is critical to verify on the ground that the PCDU operates efficiently and effectively throughout these environmental and operational changes. An SAS enables scientists and engineers in the lab to

replicate an entire satellite li fecycle by implementing dif ferent I-V curves that correspond to changing conditions. Environmental conditions include temperature and irradiance, among other factors. As FDQ�EH�VHHQ� LQ�)LJ����D�VRODU�SDQHO¶V�RXWSXW�SRZHU�ZLOO�

reduce as the temperature increases. Notice that the value for open circuit voltage (Voc) decreases dramatically as temperature rises. This is caused by increased conductivity in the semiconductor material in each cell, which lowers the junction electric field, inhibiting charge separation and effectively lowering the voltage. This is offset slightly by higher mobilit y in the electrons caused by higher temperature, but the overall effect is a reduction in eff iciency.

Figure 1. Output power dependence on temperature.

Small change in Isc, large change in Voc. As a contrast, changes in irradiance have a large impact on short circuit current (Isc) and a small impact on Voc, as seen in Fig 2. This is the result of the increased density of photons incident on the solar panel when radiation increases. In space, the incident radiation is dependent on distance from the sun, angle of arrival, and shading caused by spacecraft rotation or celestial bodies. As a PCDU designer, it is important to choose test equipment that includes the abilit y to program different curves to represent dif ferent environmental and operational phenomena. In practice, make sure the speed of the curve change and dwell time settings provide suff icient performance to emulate eclipse and axial spin conditions.

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Proc. ‘9th European Space Power Conference’, Saint Raphaël, France,

6–10 June 2011 (ESA SP-690, October 2011)

Figure 2. Output power dependence on irradiance.

Pmp = power, Imp = current, Vmp = voltage at maximum power point.

1.2 Limitations The main limitation of using an electronic instrument to simulate the output of a solar panel is the speed at which transitions can occur resulting from stimuli. A semiconductor reacts almost instantaneously to changes in load and external input. An electronic device relies on the bandwidth of its interconnected feedback loops to recreate this dynamic performance. A good rule of thumb is to ensure that the update rate of the simulator is at least one order of magnitude higher than the update rate of the APR in the PCDU. This wil l increase the likelihood that the APR sees a smooth I-V output curve rather than discrete steps. Keep in mind that update rate can take on two different forms, as seen in Fig. 3: 1. How fast the SAS changes curves to simulate a

change in operating condition 2. How fast the SAS moves its operating point along a

single curve to simulate a change in loading

Figure 3. I-V curve versus operating point

Dynamic performance requirements depend also on the regulation topology implemented in the APR. For Maximum Peak Power Tracker (MPPT) applications, it is important that the SAS utili ze a high resolution curve (many I-V points) and have suff icient loop gain EDQGZLGWK�DURXQG�WKH�³NQHH�´��6KRZQ�E\�WKH� Imp,Vmp point in Fig. 2) For Sequential Shunt Switching (S3R) applications, it is important to have a very fast current settling time and be able to support switching frequencies in the tens of kHz. 1.3 Dynamic Performance For both MPPT and S3R applications, the dynamic performance demands are such that standard DC SURJUDPPDEOH� SRZHU� VXSSOLHV� FDQ¶W� UHDFW� TXickly enough. They are designed to be very good voltage sources with high output capacitance and very distinct modes of operation between Constant Current (CC) and Constant Voltage (CV). An SAS, on the other hand, is designed to act li ke a fast-moving current source with low output capacitance and li ttle to no mode crossover effects. The best measure of SAS performance, then, is how closely it mimics the actual characteristics of a physical solar cell, module, panel, or array. 2. OPERATIONAL EXAMPLES 2.1 Voltage and current response Figs 4a-4d show the time domain response of an Agilent E4360A SAS compared to a Raloss SR30-36 solar panel with the following typical electrical characteristics provided by the manufacturer at irradiation of 1kW/m2 and 25oC panel temperature: Isc = 1.91A Voc = 21.81V Imp = 1.76A Vmp = 17.49V In this experiment, the solar panel was il luminated with an artificial li ght source capable of producing irradiation of approximately 500W/m2, similar to the solar density on the surface of Mars. 7KH�SDQHO¶V�RXWSXW�YROWDJH�ZDV�measured in response to sweeping the output current with a voltage-fed converter. Then the output current was measured in response to sweeping the output voltage with a current-fed converter. [1] The time period of the entire back and forth sweep was roughly 2 seconds. The panel was then replaced with the SAS and the same measurements repeated. During the testing, the SAS was programmed to the following settings, which were measured from the reference panel at a temperature of 44oC: Isc = 1.035A Voc = 19.225V Imp = 0.947A Vmp = 15.825V

purpose test components can all be integrated into the rack to help guard against damage caused by temperature, current, voltage, vibration and shock, and operator error.

4. CONCLUSION Satellites are high complexity machines that require sophisticated test equipment and procedures. Solar Array Simulators are DC power supplies designed specif ically to act li ke current sources and emulate the dynamic I-V output characteristics of a real photovoltaic array. Great care must be taken when considering what kind of SAS to procure, how to integrate it into a system, and how to protect your investment in the spacecraft as well as the EGSE. 5. REFERENCES

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2. Feng, Xiaogang, Liu, Jinjun, Lee, Fred. (2002). Impedance Specifications for Stable DC Distributed Power Systems. IEEE Transactions on Power Electronics Vol 17 No2.

3. Middlebrook, R. D. (1976) Input filter consideration in design and application of switching regulators. Proceedings of the IEEE Industrial Application Society Annual Meeting.

4. Roeder, Reinhard. (2010). BepiColombo Electrical Power Subsystem Specification BC-ASD-SP-00028, European Space Agency BepiColombo Satellite Programme. EADS Astrium, Friedrichshafen, Germany.

5. Seipel, Win. (2008). Sequential Shunt Regulation Application Note 5989-9791EN. Agilent Technologies, Budd Lake, NJ, USA.

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